Fans of this type must satisfy various requirements, including: low noise level, high efficiency, compactness, capacity to achieve good pressure and flow rate values.
Patent EP-O 553 598, by the same Applicant, presents a fan with blades delimited at the leading edge and trailing edge by two curves which are two circular arcs when projected in the fan plane of rotation.
Fans constructed in accordance with said patent provide good efficiency and low noise, but have limits as regards the possibility of achieving high pressure values, since the blades are made with profiles whose centre line is relatively short compared with the blade radial extension.
Moreover, fans constructed in accordance with the above-mentioned patent have a limited axial dimension, but a relatively large diameter.
For the exchanger units of heating and/or air conditioning systems for the interior of motor vehicles the overall dimensions of the fan must be limited, which means that the diameter must also be limited, whilst good air flow rates are required with high pressure and low noise.
For these reasons, in the above-mentioned exchanger units centrifugal fans are often used, which may have a relatively small diameter, but with a rather large axial dimension.